In order to maintain continuity of operation during periods of primary power source failure, many electrical systems, such as the public telephone network, utilize electrolytic batteries as interim sources of operating power. Depending upon the support voltage required, a number of battery cells are arranged in a series-connected string and the string is placed in the system circuit in parallel with the primary rectifier power source. During normal operation of the system, the rectifier source supplies operating power to the system and provides a continual "float" voltage to the cells to maintain the charge that will be needed to support the system in the event of failure of the primary power source.
The common lead-acid battery cell (PbO.sub.2 /Pb/H.sub.2 SO.sub.4) widely used in support power supplies has a useful life of a number of years; however, this term is limited by the corrosion occurring, particularly at the positive cell electrode, due to the oxidizing conditions resulting from the repeated application of charging current to the cell. While this effect is exaggerated during periods of recharging after substantial support usage, even the low-level float voltage contributes to significant oxidation of the metallic lead grid at the positive electrode with resulting buildup of oxides that will ultimately cause physical damage to the cell.
Attempts have been made previously to minimize the deleterious effects of charging current on battery cells, but none have resolved the problem of damaging oxide generation. For example, in U.S. Pat. No. 4,614,905 a shunting circuit is used to divert charging current from the more fully-charged cells of a series string to prevent direct damage from overcharging, but even this extensive control does not affect the float or trickle charge to cells exhibiting a nominal full charge, and the oxidation of the positive electrode persists. Mechanical approaches, such as suggested in U.S. Pat. No. 4,358,892, have also been proposed, but have resulted in significant cost increases while only temporarily delaying the deep oxidation of the positive electrode occurring upon the application of recharging currents and under continuing oxidizing conditions resulting from persistent float voltage.